Relax email header requirements (#20)

`git am` will accept patches which aren't fully RFC-compliant, as long
as they start with `From` and have a `Subject` line. Relax
ParsePatchHeader() so that it will accept such patches as well.

Specifically, it will tolerate patches of the form:

    From: <[email protected]>

In this case, the `Author` field will contain `[email protected]
<[email protected]>`.  Duplicate this behavior.

Signed-off-by: George Dunlap <[email protected]>
Co-authored-by: George Dunlap <[email protected]>

Author: gwd

gitdiff/patch_header.go

@@ -13,8 +13,9 @@ import (
 )
 
 const (
-	mailHeaderPrefix   = "From "
-	prettyHeaderPrefix = "commit "
+	mailHeaderPrefix        = "From "
+	prettyHeaderPrefix      = "commit "
+	mailMinimumHeaderPrefix = "From:"
 )
 
 // PatchHeader is a parsed version of the preamble content that appears before
@@ -210,6 +211,9 @@ func ParsePatchHeader(s string) (*PatchHeader, error) {
 	switch {
 	case strings.HasPrefix(line, mailHeaderPrefix):
 		return parseHeaderMail(line, r)
+	case strings.HasPrefix(line, mailMinimumHeaderPrefix):
+		r = bufio.NewReader(strings.NewReader(s))
+		return parseHeaderMail("", r)
 	case strings.HasPrefix(line, prettyHeaderPrefix):
 		return parseHeaderPretty(line, r)
 	}
@@ -368,9 +372,11 @@ func parseHeaderMail(mailLine string, r io.Reader) (*PatchHeader, error) {
 
 	h := &PatchHeader{}
 
-	mailLine = mailLine[len(mailHeaderPrefix):]
-	if i := strings.IndexByte(mailLine, ' '); i > 0 {
-		h.SHA = mailLine[:i]
+	if len(mailLine) > len(mailHeaderPrefix) {
+		mailLine = mailLine[len(mailHeaderPrefix):]
+		if i := strings.IndexByte(mailLine, ' '); i > 0 {
+			h.SHA = mailLine[:i]
+		}
 	}
 
 	addrs, err := msg.Header.AddressList("From")
@@ -380,7 +386,7 @@ func parseHeaderMail(mailLine string, r io.Reader) (*PatchHeader, error) {
 	if len(addrs) > 0 {
 		addr := addrs[0]
 		if addr.Name == "" {
-			return nil, fmt.Errorf("invalid user string: %s", addr)
+			addr.Name = addr.Address
 		}
 		h.Author = &PatchIdentity{Name: addr.Name, Email: addr.Address}
 	}

gitdiff/patch_header_test.go

@@ -289,6 +289,36 @@ CC: Joe Smith <[email protected]>
 				BodyAppendix: expectedBodyAppendix,
 			},
 		},
+		"mailboxMinimalNoName": {
+			Input: `From: <[email protected]>
+Subject: [PATCH] A sample commit to test header parsing
+
+The medium format shows the body, which
+may wrap on to multiple lines.
+
+Another body line.
+`,
+			Header: PatchHeader{
+				Author: &PatchIdentity{expectedIdentity.Email, expectedIdentity.Email},
+				Title:  expectedTitle,
+				Body:   expectedBody,
+			},
+		},
+		"mailboxMinimal": {
+			Input: `From: Morton Haypenny <[email protected]>
+Subject: [PATCH] A sample commit to test header parsing
+
+The medium format shows the body, which
+may wrap on to multiple lines.
+
+Another body line.
+`,
+			Header: PatchHeader{
+				Author: expectedIdentity,
+				Title:  expectedTitle,
+				Body:   expectedBody,
+			},
+		},
 		"unwrapTitle": {
 			Input: `commit 61f5cd90bed4d204ee3feb3aa41ee91d4734855b
 Author: Morton Haypenny <[email protected]>